New detections of Ly-alpha emission in young galaxies.

Abstract

We report IUE observations of two extremely low-metallicity H II galaxies. Ly α emission was detected in both galaxies. Our results confirm previous findings that young or unevolved galaxies exhibit weak or absent Ly α emission combined with a strong UV continuum, and that the Ly α/Hβ ratio and metal content are correlated in the sense that the lower the abundance, the larger the ratio. This supports the hypothesis that in the higher metallicity systems Ly α is destroyed by dust absorption. At redshifts of between 2 and 3, Ly α emission comparable to the strongest detected in H II galaxies [L(Lyα)~ 5 X 10^42^ erg s^-1^] would be barely visible; this fact can explain the relative lack of success in detecting Ly α emission associated with low-metallicity damped Ly α systems in QSOs. Our results cast doubt on claims of detection of high-redshift young galaxies associated with active galaxies, or with strong Ly α and both C IV and He II, since they may represent very extended narrow-line regions photoionized by radiation with a power-law spectrum rather than by normal hot stars.